Followings are a few reasons for the great interest in stoma:
Stoma provides characters of taxonomic significance. This is due to the fact that genera and even families show great constancy for their possession of a particular stomatal complex.
The study of stomatal complexes becomes very useful to understand the evolutionary relationship of plants when the classical methods of cytology and genetics etc. cannot be applied. Apart from stomatal complex surface view of guard cells and the aperture have taxonomic importance.
The aperture is unique in Azolla where it is oriented at 90° to guard cells. The frequency and distribution of stoma (epistomatic, hypostomatic or amphistomatic) form different taxonomic groups. Appearance of stoma in surface view and their ontogeny provide characters not only to understand the evolution of stomata but also form the bases of different classificatory systems.
Stomata are most important portals for controlled access and egress of gases. Stomata regulate entry of carbon dioxide thus controlling the rate of photosynthesis. In crop plants photosynthesis is a major factor in determining the rates of dry matter accumulation. Stomata also control the transpirational water loss and it thus regulates the entry of materials into root and transport materials within the plant.
Stomata open and close due to the movements of guard cells thus regulating gas exchange. Stomata appear to open and close in response to accumulation of potassium ion. During opening potassium ion accumulates in guard cells.
Potassium ion moves to subsidiary cells during closing. It is observed that there is a shuttle of potassium ion between guard cells and subsidiary cells in grasses. There are many more biochemical reactions that result in opening and closing of stoma and it is a fascinating subject of investigation.
Stomata can sense small changes in carbon dioxide concentration and respond accordingly. A change in carbon dioxide concentration in atmosphere sets in motion a series of biochemical reactions in guard cells.
This causes the movement of guard cells thus opening and closing of stomata. This property of stomata can be utilized in determining the air quality in the atmosphere. Sen and De (1992b) states that ‘the attributes of stomata of certain fern taxa can be used as biomonitors of air-pollution’.
The study of ontogeny of stoma and stomatal complex has greatly improved our knowledge about the establishment of cell polarity, development, differentiation and pattern formation over leaf surface.
Another aspect of stoma is proving to be a fascinating subject for investigation. The stomata are the mode of ingress of some economically important plant pathogens. Ex. Puccinia graminis (responsible for rust disease of wheat), Plasmopara viticola (responsible for downy mildew of Vitis vinifera) and Pseudoperonospora humuli (responsible for downy mildew of Humulus lupulus) etc.
The nature of stimulus that attracts fungal hyphae/zoospores to stomatal aperture has received considerable attention. A number of explanations have been forwarded but scarcely conclusive. The host and pathogen interaction lead to the production of phytoalexin by host and phytotoxin by pathogen.
Phytoalexin and phytotoxin both affect stomatal movement. As for example, pisatin — a phytoalexin produced by peas inhibits stomatal opening in peas. Victorin — a phytotoxin produced by Helminthosporium victoriae causes stomatal closure.
Stomata are also proving to be the subject of investigation of antitranspirants. Antitranspirants are used to close stomata enabling a crop plant to withstand a period of drought. Due to closure of stomata photosynthesis is prevented thus reducing yield.
Transpirational water loss is also prevented. Perennial crops are subjected to such treatment for a particular season when their survival becomes more important than good yields. Abscisic acid, aspirin etc. are used as antitranspirants.
Stomata are intimately linked to physiological processes that are critical for survival in terrestrial environments.
They have adapted many ecological features that are vital for survival:
i. Hydathodes:
Haberlandt (1914) introduced the term hydathode to designate the structure that secretes water. Haberlandt distinguished two kinds of hydathode-epithem hydathode and epidermal hydathode. The epidermal hydathode, in addition to water, secretes ions and minerals. They are now referred to as salt glands.
Epithem hydathode:
Generally referred to as hydathode or water stoma.
a. Distribution:
Hydathode occurs in many plant families (e.g. Poaceae, Araceae, Ranunculaceae, Papaveraceae etc.) including pteridophytes (Equisetum). They may occur over the entire surface of the leaf or may be restricted to margins or tips.
They are also observed at the tip of tendrils (e.g. Vitis vinifera). They are reported in plants of various life forms and of various ecological habits. Ex. Lathrnea (a total parasitic plant), Crassula argentata (succulent leaf), Ranunculus fluitans (submerged leaves) etc.
b. Structure:
A typical hydathode consists of:
(a) Water pore,
(b) Epithem, and
(c) Tracheid (Fig. 12.18).
(a) Water pore:
The continuity of the epidermis at the hydathode region is interrupted by means of opening – termed water pore. These water pores are generally regarded as modified stomata, which are formed by guard cells. The guard cells lack chloroplasts and contain large nuclei and mitochondria, which are distributed throughout the cytoplasm. A large part of the protoplast is occupied by vacuum.
The guard cells are unable to perform the closure movements and so the water pores always remain open. In this context, it may be mentioned that the water pores of Impatiens and Tropaeolum are active. The opening of the water pores of Impatiens is observed in presence of light.
The water pores lead to a chamber termed sub-stomatal chamber. One or two layers of cells bound the chamber. Each cell is more or less isodiametric and contains large nucleus, large vacuole and mitochondria, which are localized mainly around the nuclei. The cells are larger than the neighbouring epithem cells.
(b) Epithem:
It consists of parenchyma tissues, which are situated below the sub-stomatal chamber. The cell wall is thin and the cell lacks chloroplastids. In some cells fragmented and reduced vacuole is present.
Electron microscopic study with the epithem cells of Taraxacum officinale, Papaver rhoeas reveals the presence of highly developed wall protuberances in them. These cells are known as ‘transfer cell’. These cells contain dense cytoplasm with mitochondria, endoplasmic reticulum, ribosomes and polysomes.
A sheath of cells completely or incompletely surrounds the epithem of many dicots. The sheath of Saxifraga consists of two layers and the cell walls are suberized. The cells may contain tanniniferous substances. Abundant intercellular spaces occur in the epithem except Crassula argentata. These spaces are sometimes equal to the size of a cell.
(c) Tracheid:
The tracheids often abut on the intercellular spaces of the epithem. The epithem may have single, double or even triple vein supply. The veins fuse with each other before reaching the epithem.
The terminal tracheids may extend through the epithem up to the sub-stomatal chamber (e.g. Primula vulgaris). In Apomogeton distachyus the vascular bundles extend up to the opening of the hydathode, as a result the surface layer may be torn. The terminal tracheids usually have spiral, scalariform or annular thickenings.
Function:
The main function of hydathode is guttation, i.e. exudation of water in liquid form. A single leaf of Colocasia may guttate 10ml — 100ml of water in a single night. It is generally agreed that guttation occurs as a result of root pressure.
The guttation fluid may be composed of pure water or a dilute solution of ammonium phosphate, magnesium chloride, calcium nitrate etc. The occurrence of amino acids, sugars and water-soluble vitamins in the fluid are reported in the leaves of Zea mays and Colocasia antiquorum.
An altogether different view was expressed by Voronin et al.. (1976) concerning the function of the hydathodes of Crassula, though no experimental evidences have been cited.
According to them the hydathodes, in addition to guttation, absorb atmospheric water from the dew. They observed that, in the species of Crassula, which grow in dry habitats, hydathodes generally occur on the exposed surface of leaves that led them to suggest this interesting view.
Salt gland:
Salt glands occur in several angiospermic families (Plum- baginaceae, Tamaricaceae etc.) and in the some species of Acanthaceae, Chenopodiaceae, Primulaceae, and Gramineae etc. They also occur in mangrove plants (e.g. Avicenma, Aegiceras etc.). In contrast to epithem hydathodes these glands have no direct connection with the vascular elements and do not possess water pores.
Salt glands may be composed of two cells (e.g. Spartina) or many cells (e.g. Acanthus, Aegiceras, Limonium etc.) Fig. 12.19. The salt glands of Atriplex have hairs that consist of a distended head bladder cell with one celled or 2-3 celled uniseriate stalk (Fig. 12.20).
Salts are stored in these glands and may be exuded by guttation flow. These glands play an important role in the regulation of mineral contents of plants.
Function of epidermis:
(i) The thick cuticularized epidermal cells protect the inner tissues from adverse natural calamities, resist attack of insects and prevent the entry of pathogen.
(ii) Cuticularized epidermis prevents cuticular transpiration.
(iii) Guard cells of stomata, the chloroplast containing modified epidermal cells can photosynthesize.
(iv) The specialized epidermal cells —the bulliform cells help in water storage, unrolling of developing leaves and the opening and closing movements of mature leaves.
(v) Velamen of orchid root and epiblema with root hairs help in absorption.
(vi) The epidermis is the storage site for water, unwanted material (salt glands) and food.
(vii) The watery vacuoles, present in the epidermal cells, absorb ultraviolet and infrared radiation and thus protect the plants from their harmful effects.
(viii) The wax deposition, suberization, cutinization etc. represent the process of secretion, which in certain cases occur in the epidermis.
(ix) The epidermal cells can perceive the external stimuli and can react accordingly.
Diagram illustrating stomatal apparatus in transverse section.
